Fingerprint sensing and matching is widely used for personal identification or verification. A common approach to fingerprint identification involves scanning a sample fingerprint or an image thereof and storing the image and/or unique characteristics of the fingerprint image. The characteristics of a sample fingerprint may be compared to information for reference fingerprints already stored in a database to determine or verify identification of an individual.
A fingerprint sensor is an electronic device used to capture a digital image of the fingerprint pattern. The captured image is called a live scan. This live scan is digitally processed to create a biometric template (a collection of extracted features) which is stored and used for matching. There are four types of more commonly used fingerprint sensor technologies: optical, ultrasonic, radio frequency (RF), and capacitance.
Optical fingerprint sensing involves capturing a digital image of the print using visible light. This type of sensor is essentially a specialized digital camera. In one version, the top layer of the sensor, where the finger is placed, is referred to as the touch surface. Beneath this layer is a light-emitting phosphor layer which illuminates the surface of the finger. The light reflected from the finger passes through the phosphor layer to an array of solid state pixels (a charge-coupled device) which captures a visual image of the fingerprint. This type of sensor has the disadvantage that the imaging capabilities are affected by the quality of skin on the finger. For example, a dirty or marked finger is difficult to image properly. Further, it is possible for a person to erode the outer layer of skin on the fingertips to the point where the fingerprint is no longer visible. The sensor can also be easily fooled by an image of a fingerprint if not coupled with a “live finger” detector. However, unlike capacitive sensors, this sensor technology is not susceptible to electrostatic discharge damage.
An ultrasonic sensor makes use of the principles of ultrasonography in order to create visual images of the fingerprint. The ultrasonic sensor uses very high frequency sound waves to penetrate the epidermal layer of skin. The sound waves are generated using piezoelectric transducers and reflected energy is also measured using piezoelectric materials. Since the dermal skin layer exhibits the same characteristic pattern of the fingerprint, the reflected wave measurements can be used to form an image of the fingerprint. This eliminates the need for clean, undamaged epidermal skin and a clean sensing surface.
A radio-frequency (RF) fingerprint sensor is capable of using RF electric fields to develop an electronic representation of the fingerprint pattern. Such a device can be fabricated as a standard CMOS integrated circuit on a monocrystalline silicon substrate. This process allows the electronic structures required of reading the signal from each of the sensor's pixels or sensing electrodes to be fabricated directly beneath the pixels. Locating the signal conditioning electronics or sense amps under pixel was essential to achieving adequate performance of the circuitry. One such RF fingerprint sensing device is disclosed in U.S. Pat. No. 5,940,526 to Setlak et al. The patent discloses an integrated circuit fingerprint sensor including an array of RF sensing electrodes to provide an image of the fingerprint friction ridges and valleys. More particularly, the RF sensing permits imaging of live tissue just below the surface of the skin to reduce spoofing. In a follow-up patent application, US Pat. Pub. No. 2013/0181949 (Jul. 18, 2013), Setlak discloses an improved fingerprint sensor, which includes pixels, pixel sensing traces each associated with a respective pixel, and electrodes overlying the pixel sensing traces. The finger sensor may also include pixel sensing circuitry coupled to the pixel sensing traces and the electrodes.
A DC capacitance sensor uses capacitance principles to develop fingerprint images. In this method, the sensor array pixels each act as one plate of a parallel-plate capacitor, the dermal layer (which is electrically conductive) acts as the other plate, and the non-conductive epidermal layer acts as a dielectric. A passive capacitance sensor uses the principle described above to form an image of the fingerprint patterns on the dermal layer of skin. Each sensor pixel is used to measure the capacitance at that point of the array. The capacitance varies between the ridges and valleys of the fingerprint due to the notion that the space between the dermal layer and the sensing element in valleys contains an air gap. The dielectric constant of the epidermis and the area of the sensing element are known values. Hence, the measured capacitance values can be used to distinguish between fingerprint ridges and valleys.
An active capacitance sensor uses a charging cycle to apply a voltage to the skin before measurement is conducted. The applied voltage charges the effective capacitor. The electric field between the finger and the sensor follows the pattern of the ridges in the dermal skin layer. During the discharge phase, the voltage across the dermal layer and the sensing element is compared against a reference voltage and the capacitance is computed. The distance values are then calculated and used to form an image of the fingerprint. An active capacitance sensor measures the ridge patterns of the dermal layer like the ultrasonic method. This method obviates the need for clean, undamaged epidermal skin and a clean sensing surface. It may be noted that an active capacitance sensor can makes use of a RF excitation to replace the DC voltage charging.
An example of a capacitance-based fingerprint sensing device is disclosed in U.S. Pat. No. 5,325,442 to Knapp. This device has a row/column array of sensing elements which are coupled to a drive circuit and a sensing circuit by sets of row and column conductors, respectively. The sensing elements are actively addressable by the drive circuit. Each sense element includes a sense electrode and a switching device, such as a thin film transistor (TFT) switching device, for actively addressing that sense electrode. The sense electrodes are covered by an insulating material and are for receiving a finger. Capacitances resulting from individual finger surface portions in combination with sense electrodes are sensed by the sense circuit by applying a voltage to the sense electrodes and measuring charging characteristics.
For detailed configurations of capacitive fingerprint sensors and related circuitry design, please consult the following additional examples: U.S. Pat. No. 8,564,314 (Oct. 22, 2013) issued to J. Shaikh, et al.; US Pat. Pub. No. 2013/0181949 (Jul. 18, 2013) by Setlak; U.S. Pat. No. 8,736,001 (May 27, 2014) issued to M. Salatino, et al.; U.S. Pat. No. 8,766,651 (Jul. 1, 2014) to M. H. Kang, et al.; U.S. Pat. No. 8,772,884 (Jul. 8, 2014) to R. H. Bond, et al.
Current fingerprint sensors are typically implemented on a rigid and brittle substrate (e.g. Si wafer or inorganic glass) having most or all of the sensor components being rigid and/or brittle as well. These rigid components are incompatible with flexible electronics (e.g. the bendable flexible smart phone or wearable device). For instant, in a commonly used design, the fingerprint sensor subsystem contains a laser-cut sapphire crystal plate that fits into a stainless steel detection ring, which in turn is in physical connection with a touch ID sensor and a tactile switch. All these individual components are very rigid and unfit for a flexible device design.
However, the flexible substrate (polymer) based fingerprint sensor has its own intrinsic drawbacks as well. For instance, the polymer, such as polyimide (PI), does not exhibit an adequate scratch or abrasion resistance, which is required of a fingerprint sensor that experiences repeated relative motions/contacts between a finger and a polymer surface. The polymer substrate must also have good mechanical integrity as well as good electrical conductivity (i.e. reduced sheet resistance). This electrical characteristic is essential to form a sensitive and selective capacitance measuring pixel. However, most of the intrinsically conductive polymers are not mechanically robust, sufficient for use as a substrate of a flexible sensor. Most of the mechanically strong polymers are not electrically conductive.
Thus, it is an object of the invention to provide a flexible substrate and related sensor components deposited thereon for use in a biometric sensor, such as a fingerprint sensor. These sensor components, individually or in combination, must be mechanically robust, scratch-resistant, and electrically conducting for reduced impedance and improved sensitivity of the sensor. However, where a finger is touched or swiped, the surface must be electrically insulating and such a skin layer must be as thin as possible. All these seemingly conflicting requirements make the design and production of an assembly containing these components a formidable task.
Hence, a specific object of this invention is to provide a layer of electrically conductive material that is capable of well-adhering to a flexible substrate, and a protective hard coating layer covering this layer of electrically conductive material, resulting in a flexible laminate that meets all the technical requirements of a flexible fingerprint sensor unit or subassembly.